In many areas of industry, there is a need for conductive pigments which make it possible to produce, for example, plastics, paints, coatings or fibers which are electrically conductive, antistatic, or shield off electromagnetic waves. For this purpose, conductive carbon black is used in large amounts but cannot be employed for transparent, light-colored or colored coatings due to its high light absorption in the visible region of the spectrum. Another problem is the strong absorption of carbon black in the IR region, as a result of which the coated articles heat up when exposed to insulation, for example, which is often undesirable. In the case of light-colored coatings, nickel-coated graphite, metal platelets and mixed oxides, for example, antimony-doped tin oxide, are therefore used. The mixed oxides mentioned may also have been deposited on supports, for example, on platelet-like mica or spherical barium sulphate.
U.S. Pat. No. 4,568,609 describes conductive platelet-like pigments in which a platelet-like substrate has been overcoated with a conductive layer of antimony-doped tin oxide.
The conductive layer is applied to the substrate by metering salts of zinc and antimony at a predetermined mixing ratio and a suitable pH to an aqueous suspension of the substrate in such a manner that in each case hydrolysis and deposition of the hydroxides or hydrated oxides on the platelet-like substrate take place immediately. After coating is complete, the pigment is separated off, washed and dried and calcined at 650.degree. to 850.degree. C.
The preparation of a substrate-free conductive pigment is described in EP-A-0,441,427. This is done by mixing the alcoholic solution of a tin salt with the aqueous solution of a fluoride, separating off, drying and calcining the precipitated tin hydroxide at 500.degree. C.
Apart from the wet-chemical preparation of conductive mixed oxides, electrically conductive mixed oxides can also be prepared by a dry high-temperature treatment. U.S. Pat. No. 4,655,966 describes the preparation of antimony-doped tin oxide by intimate mixing of tin dioxide and antimony trioxide in a ratio of 1.25 to 10 parts by weight of antimony oxide to 100 parts by weight of tin dioxide, followed by heating the resulting mixture to 900.degree. to 950 .degree. C.
The nearly white pigment is used for electrically conductive white or colored coatings of aircraft, in order to prevent an accumulation of static electricity on electrically non-conductive parts. However, varnishes such as those selected from the group consisting of acrylate resin, alkyd resin, amino resin, polyester resin, polyurethane resin and epoxy resin, containing the conductive pigment can also be used in the electronics industry.
EP-A-0,487,366 describes antistatic and radio-transparent coatings for antennas of space satellites. The pigment used for this purpose consists of tin dioxide and titanium dioxide which are both doped with antimony. They are prepared by intimate mixing of 30 to 73 parts of SnO.sub.2, 70 to 27 parts of TiO.sub.2 and 0.1 to 10 parts of Sb.sub.2 O.sub.3, followed by calcining of the mixture at 700 to 1,000.degree. C. for 5 to 10 hours. Alternatively, in order to improve the degree of whiteness of the paint, a non-conductive white pigment, for example TiO.sub.2, ZnO, Zn.sub.2 TiO.sub.4, ZnSnO.sub.4 or SnTiO.sub.4, can be added to the paint.
The values given for the surface resistance of the coating are 2-500 M.OMEGA. per unit area. The conductivity of this paint is too low for antistatic coatings for which a surface resistance of less than 1 M.OMEGA. is usually required.